380 STUDIES IN EVOLUTION 



by the circumbonal tract. With the approach of maturity 

 this gradually disappears, and at full growth every trace of 

 it has become obliterated. 



Spirifer crispus Hisinger, 1826. 



(rLATE XX, figures 6, 7.) 



Spirifer crispus, var. simplex Hall, 1879. 



(Plate XX, figures 4, 5.) 



Retieularia hicostata Vanuxem, 1842, var. petila 

 Hall, 1879. 



(Plate XX, figures 1-3.) 



Spirifera crispa Hall. Twenty-eighth Ann. Rapt. N. Y. State Mus. Nat. 



liist., p. 157, pi. 24, figs. 8-12, 19, 1879. 

 Hall. Eleventh Ann. Kept. State Geol. Indiana, p. 295, pi. 24. 



figs. 6-12, 19, 1882. 

 Spirifera crispa, var. simplex Hall. Twenty-eighth Ann. Rept. N". Y. 



State Mus. Nat. Hist., p. 157, pi. 24, figs. 1-5, 1879. 

 Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 286, 



pi. 24, figs. 1-5, 1882. 

 Spirifera bicostaiaf var. petila Hall. Trans. Alb. Inst., vol. x, abstract, 



p. 15, 1879. 

 Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 297, 



pi. 27, figs. 8, 9, 1882. 



The three forms which are here treated together are closely 

 allied in all their general characters. It is in their initial 

 stages, however, that the resemblance becomes more than 

 superficial, for, in young shells of less than 2 mm. in length, 

 it is difficult, and sometimes impossible, to refer them to any 

 one of the three groups. A general expression of the com- 

 mon characters is furnished by the young of Spirifer crispus, 

 var. simplex, illustrated by figure 4, on Plate XX. 



Taking Retieularia hicostata, var. petila, as the simplest 

 form, the young shell is found to be nearly circular in outline, 

 with a single, broad, median fold on the dorsal valve. Pass- 

 ing to S. crispus, var. simplex, of the same size, the outline 



